I. Field of the Invention
The present invention relates to an interface for a digital wireless telephone system. More particularly, the present invention relates to a digital network interface that is compatible with standard analog fax machines.
II. Description of the Related Art
Analog facsimile (fax) machines transmit digital data representative of paper documents over analog transmission systems by converting the digital data into sinusoidal tones. FIG. 1 is a block diagram of two fax machines 2 coupled via an analog transmission system 4. Generally, analog transmission system 4 is the public switched telephone network (PSTN), which is the wire line telephone network used to provide conventional telephone service.
Increasingly, data transmission is being performed via the use of digital transmission systems such as the Internet in addition to, or instead of, analog transmission systems. FIG. 2 is a block diagram of two fax machines 2 coupled via an analog transmission system 8 and a digital wireless transmission system 6.
One particularly important type of digital network is a digital wireless cellular telephone system which uses digital signal, processing and digital communication techniques to provide efficient wireless telephone service using radio frequency (RF) signals. FIG. 3 is a block diagram of a typically configured digital cellular telephone system. Subscriber units 10 and 11 (usually cellular telephones) interface with base stations 12 via the use of digitally modulated RF signals, and base station controller 14 provides various call management functionality to allow mobile communications to be conducted.
Additionally, FIG. 3 shows subscriber unit 10 in communication with two base stations 12 in a state referred to as soft handoff, which is consistent with the use of the IS-95 over-the-air cellular telephone system interface standard, which incorporates the use of Code Division Multiple Access (CDMA) signal processing and communications to provide highly efficient and robust cellular telephone service.
Digital transmission systems in general, and wireless digital transmission systems in particular, have substantially different transmission characteristics than analog transmission systems. These differing transmission characteristics include variable transmission delay created by transmission retry attempts and an inability to transmit tones in complete fashion because of the use of lossy encoding. Lossy encoding is performed on voice and other audio information transmitted using a digital cellular telephone system to minimize the amount of data necessary to conduct voice communication.
Additionally, the maximum data transmission rate of a voice channel in a digital wireless telephone system is much less than that of a wire base analog telephone system. Voice communication is conducted over these reduced rate channels via the use of the lossy encoding mentioned above, which is more efficient than analog systems and other non-lossy encoding techniques.
These different transmission characteristics make wireless digital transmission systems incompatible with the use of analog fax machines. For example, while voice communication can tolerate lossy encoding, analog fax transmissions cannot. Additionally, analog fax transmissions generally require higher data rate channels than those offered by wireless digital telecommunications systems.
As the cost of digital wireless telecommunications service decreases with the increased availability of RF spectrum and the introduction of more efficient digital technology, the use of digital wireless telephone systems as a primary source of telephone service will increase. For individuals and businesses that already possess analog fax machines, however, it will be desirable to continue to use analog fax machines with the digital wireless telephone. Thus, there is a need for a method and apparatus for allowing analog fax machines to conduct communication over a connection that includes a digital network, including a digital wireless telecommunications system.
The present invention is a novel and improved method and apparatus for providing an interface to a digital wireless telephone system compatible with standard analog wire line fax machines. To process a fax, a source interface waits until an interface to a destination fax machine has been established before establishing an interface to a source fax machine. The source fax interface rate must be less than or equal to the destination fax interface rate and the data rate of the digital channel. To establish the source fax interface at the proper rate, the source interface first selects an initial data rate from a set of standard fax transmission rates. The source interface sends unacceptable rate (failure to train) messages to the source fax machine until the source fax interface rate is less than or equal to the data channel rate and the destination fax interface rate. During fax processing, the destination interface inserts non-printed data (fill bits) if the destination fax transmission rate is greater than the source fax transmission rate.